Snom 4S NAT Filter Admin Manual page 17

• When refresh packets cannot be delivered because of congestion
or other network problems, the NAT router will close the ports and
there is no way to open them again from the outside. If the user
agent refreshes the bindings, the port is opened automatically
again.
• User agents may send from time to STUN packets and process the
answer to check if the IP address has changed. In this case, the user
agent may refresh the SIP binding.
A user agent that supports this way of refreshing the bindings
includes a "P-NAT-Refresh" header in the REGISTER message:
REGISTER sip:snom.com SIP/2.0
Via: SIP/2.0/UDP 192.168.1.10:5060;branch=z9hG4bK-fozdn9kbolfw
From: "Karl Klammer" <sip:kk@snom.com>;tag=9e9mynnnwa
To: " Karl Klammer" <sip:kk@snom.com>
Call-ID: 10f2c240790b-cj4sy7drgp6q@192-168-1-10
CSeq: 2 REGISTER
Max-Forwards: 70
Contact: <sip:kk@192.168.1.10:5060;line=5zy4hsui>;q=0.7
User-Agent: snom200-2.05h
P-NAT-Refresh: 15
Supported: gruu
Expires: 86400
Content-Length: 0
The NAT Filter will answer this request with a P-NAT-Header:
SIP/2.0 200 Ok
Via: SIP/2.0/UDP 192.168.1.10:5060;branch=z9hG4bK-iu90avx18fc6;rport
=34142;received=217.231.163.127
From: "Karl Klammer" <sip:cs@snom.com>;tag=9e9mynnnwa
To: "Karl Klammer" <sip:cs@snom.com>;tag=996wctfnen
Call-ID: 2605c340c91d-cj4sy7drgp6q@192-168-1-10
CSeq: 2 REGISTER
Contact: <sip:kk@192.168.1.10:5060;line=5zy4hsui>;expires=3600;gruu=
"sip:kk@snom.com;gruu=5npko91p"
Server: snom proxy (Unix) 2.42.7
Date: Sun, 06 Jun 2004 11:50:22 GMT
P-NAT-Refresh: 15
Content-Length: 0
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snom technology AG • 17